To combat extinction, butterflies evolve fast

CHICAGO -- When faced with extinction, butterflies on two South Pacific islands quickly developed genetic defenses that helped them fight back, a team of researchers has found.

It said the change in the butterflies was the fastest example of natural selection ever observed and showed that evolution can happen quickly when the stakes are high.

In 2001, male Hypolimnas bolina butterflies on the Samoan islands of Savaii and Upolu were extremely rare. Just 1 percent of these butterflies -- known commonly as Blue Moon or Great Eggfly -- were male. They were under attack by the Wolbachia bacteria, a parasite passed down through the female that kills male butterflies before they can hatch.

By last year, however, the numbers of males were approaching those of females. They were helped by development of a genetic mutation that suppresses the bacteria. "This is one of the most clear and fastest cases of evolution under natural selection," said Sylvain Charlat of University College London, whose study appears in the journal Science.

To test whether the male butterflies' resurgence was due to genetic changes in the butterflies or changes in the parasite, the team bred infected female butterflies with butterflies from a different island that did not have the genetic mutation. The offspring were then bred with butterflies from a noninfected island to dilute the gene that suppressed the parasite.

"After we did that for three generations, we came back to complete male killing. . . . That demonstrated that the observed pattern was due to suppression and not due to another phenomenon," Charlat said.

The researchers are not sure whether the gene that suppressed the parasite emerged from a mutation in the local population or was introduced by migratory Southeast Asian butterflies.

Natural selection typically moves very slowly, sometimes over hundreds of years, but when species are under severe attack, this process is accelerated.